Central Somatosensory Pathways

Question 1

What is the general route of mechanosensory afferent fibres?

Answer 1

Fast and myelinated
Enters spinal cord or brain stem
Ascend in white matter dorsal columns
Makes local connections in spinal cord
Very long

Question 2

What is the topography of dorsal white matter in the spinal cord?

Answer 2

Lower levels tend to be close to midline
- Sacral and lumbar
Thoracic level more lateral
Cervical level most lateral

Question 3

Where is the first synapse for ascending fibres?

Answer 3

Dorsal column nuclei in brain stem

Question 4

What are the other names for the dorsal column nuclei?

Answer 4

Gracile and cuneate nucleus

Question 5

Describe the main tactile mechanoreceptive pathway

Answer 5

Three neuron sequence to reach somatosensory cortex
Cell bodies of sensory fibres in dorsal root ganglia or trigeminal ganglion
- Enter dorsal column in spinal cord
- Terminate in dorsal column nuclei in caudal medulla
Decussation in caudal medulla
- Arcuate fibres cross to other side
next neurons ascend in flat bundle of fibres = medial lemniscus
- Loops around brain stem structures as it moves up
Next synapse in ventral posterolateral nucleus in thalamus
- Topography maintained
Third neuron = thalamic neuron
- Projects to somatosensory cortex

Question 6

What is the main tactile mechanoreceptive pathway called?

Answer 6

Dorsal column medial lemniscus pathway

Question 7

Where does a lot of the proprioceptive information from muscle end up in the brain?

Answer 7

Cerebellum, via spinocerebellar tracts

Question 8

What functional distinction in terms of information does the central sulcus represent?

Answer 8

Forward from central sulcus = action

Behind from central sulcus = sensation and perception

Question 9

Where is the somatic sensory cortex located?

Answer 9

Postcentral gyrus

Question 10

What is the Brodmann’s area for the primary somatic sensory cortex?

Answer 10

SI divided into (moving away from central sulcus):

• 3a
• 3b
• 1
• 2

Question 11

What part of the thalamus does face sensory information from the trigeminal nerve go?

Answer 11

Ventral posterior medial nucleus (VPM)

Question 12

What part of the thalamus does spinal sensory information go?

Answer 12

Ventral posterior lateral nucleus (VPL)

Question 13

What does the secondary somatic sensory cortex do?

Answer 13

Gets very little thalamic information

More downstream processing

Question 14

Describe the topography of the primary somatic sensory cortex

Answer 14

Parts of body that are near each other are generally represented close to each other
Map is replicated across four Brodmann areas
Huge magnification of some areas of body reflecting density of mechanoreceptors from that area

Question 15

Is the secondary somatic sensory cortex also topographically organised?

Answer 15

Yes, but not based on thalamic input

Question 16

What do the posterior parietal cortices do?

Answer 16

Represent left and right parts of body

Question 17

Which areas receiving somatic sensory information may not be as strictly somatotopic?

Answer 17

SII

2 posterior parietal cortices

Question 18

How are the two sides of the body represented in the thalamus?

Answer 18

Represented areas mirror reflections

Question 19

Is there any segregation of different sensory receptor fields?

Answer 19

Yes, there’s some degree

Eg: both slowly adapting and rapidly adapting receptor fields clustered

Question 20

Describe the connections within the brain that establish functional heirarchies

Answer 20

Thalamus
Main destination of thalamic information are 3b
Area 3b shares it with other areas
Cortical areas start to segregate features > brought back together to perceive single object - don’t know how or where
More higher order information gives information about things including:
- Grip
- Force
- Identity of object
Secondary somatosensory cortex
- Sends information onto hippocampus and amygdala
Parietal areas 5 and 7 also receive information from primary somatosensory cortex
- Sends onto motor and premotor cortical areas

Question 21

What is the hippocampus’ role in somatosensing?

Answer 21

Role in spatial memory

Question 22

What is the amygdala’s role in somatosensing?

Answer 22

Involved in emotion - unpleasant touches

Question 23

What kind of information might the parietal areas 5 and 7 be sending to the motor and premotor cortical areas?

Answer 23

More synthetic information to help with motor control

Question 24

What are the three key points of cortical organisation?

Answer 24

Topography
Magnification
Plasticity

Question 25

What happens to the area representing a finger after it has been amputated?

Answer 25

Territory taken over by remaining digits

Question 26

Does plasticity occur at all points of the dorsal column medial lemniscus pathway?

Answer 26

Yes, evidence suggests so

Question 27

What happens when regenerating nerves grow back to the wrong place?

Answer 27

Mislocalisation

No adaptation

Question 28

What is a phantom limb?

Answer 28

After a part of body has been lost, normal outcome > person still perceives missing part still present
Can be
- Complete
- Incomplete

Question 29

Why might a phantom limb happen?

Answer 29

No reason to change information in brain because it’s not being updated - brain doesn’t know information HAS been changed

Question 30

When might a phantom limb cause problems?

Answer 30

When part was lost because of trauma
Patient feels excruciating pain, as this was the last sensation the brain perceived to be in that area, and it can’t be updated